1. Field of the Invention
The present invention relates to a process for the preparation of isoalkenes by catalytic conversion of n-alkenes, catalysts which are suitable for this conversion, and a process for the preparation of such catalysts.
This invention further relates to olefin isomerization.
In one of its more specific aspects, this invention relates to selective isomerization.
2. Information Disclosure Statement
Isomerization processes can be directed towards either skeletal isomerization or double bond isomerization. Skeletal isomerization is concerned with reorientation of the molecular structure in respect to the formation or elimination of side chains. Double bond isomerization is concerned with relocation of the double bond between carbon atoms while maintaining the backbone of the carbon structure. Most isomerization processes give rise only to double bond isomerization.
It is frequently necessary to convert olefins into other olefins having a different skeletal arrangement. For example, normal butenes are converted into isobutene for polymerization, alkylation, disproportionation, etc. Similarly, normal amylenes must be converted to isoamylenes prior to dehydrogenation to isoprene.
While a number of catalytic materials possess some activity for such a conversion, not all possess sufficient selectivity to be economical. Because the feeds are generally the relatively reactive olefins, many catalysts cause undesirable side reactions such as polymerization or cracking. Moreover, some catalysts are difficult to prepare and regenerate. Additionally, some catalysts are less effective with certain olefins than with others. Consequently, there is a continuing interest in the development of new skeletal isomerization catalysts and processes to improve efficiencies and to give optimum results for various industrial requirements. A comprehensive review is provided by V.R. Choudhary in "Catalytic Isomerization of n-butene to Isobutene," Chem. Ind. Dev, pp. 32-41 (1974).
It is generally known that n-paraffins with, for example, 4 to 7 carbon atoms can be converted to the corresponding isomeric paraffins by using suitable acid catalysts in the temperature range of from 100.degree. to 250.degree. C. Examples of this process are the numerous isomerization processes used in the petrochemical and mineral oil industries for increasing the octane number of light, paraffinic mineral oil fractions. Furthermore, it is known that, in contrast to this, olefins of the same number of carbon atoms cannot be converted to the corresponding iso-olefins or can only be converted to the corresponding iso-olefins under difficult conditions, for example at very high temperatures and with poor yield. The attempts hitherto described in the literature for the direct isomerization of the skeleton of e.g. n-butene to give isobutene or e.g. of n-pentene to give isopentenes over catalysts arranged in a fixed bed are characterized by only initially high yields and selectivities, which diminish and deteriorate considerably after a short period of operation, often after only a few hours. The deterioration in the yields and selectivities is generally attributed to the loss of actively effective catalyst surface or to the loss of active centers. In addition to this, high catalyst fouling rates, formation of oligomers and cracking reactions are observed.
Thus, in U.S. Pat. No. 3,531,542, a process is described for obtaining isobutene from n-butene, in which an Al.sub.2 O.sub.3 catalyst arranged in a fixed bed is employed in a number of stages. In U.S. Pat. No. 3,663,453 the same conversion is conducted also in a fixed bed, with a catalyst consisting of zirconium oxide and an Al.sub.2 O.sub.3 /ZrOCl.sub.2 catalyst. The catalytic isomerization of olefinic hydrocarbons in a fixed bed is also described in U.S. Pat. No. 2,568,964. It is reported that carbon deposits form on the catalyst material during the isomerization process, which reduce the activity and necessitate periodic regeneration of the catalyst. It is stated that the catalyst regains its full activity after regeneration, but at least one disadvantage which remains is that the isomerization process itself has to be interrupted during the period of regeneration.
As is known, butylene or butene exists in four isomers: butene-1, cis-butene-2, its stereo-isomer trans-butene-2, and isobutene. Conversions between the butenes-2 are known as geometric isomerization, whereas those between butene-1 and the butenes-2 are known variously as position isomerization, double-bond migration, or hydrogen-shift isomerization. The aforementioned three isomers are not branched and are known collectively as normal or n-butenes. Conversion of the n-butenes to isobutene, which is a branched isomer, is widely known as skeletal isomerization. The same general terminology is used when discussing skeletal isomerization of other n-alkanes and olefins, as well as paraffinic compounds such as n-alkenes.
Isobutene has become more and more important recently as one of the main raw materials used in the production of methyl tert-butyl ether (MTBE), an environmentally-approved octane booster to which more and more refiners are turning as metallic additives are phased out of gasoline production. However, processes for the skeletal isomerization of olefins e.g., to produce isobutene, are relatively non-selective, inefficient, and short-lived because of the unsaturated nature of these compounds. On the other hand, positional and skeletal isomerization of paraffins and alkyl aromatics are fairly well established processes, in general utilizing catalysts typically comprising metallic components and acidic components, under substantial hydrogen pressure. Since paraffins and aromatics are stable compounds, these processes are quite successful. The heavier the compounds, in fact, the less severe the operating requirements.
Olefins, however, are relatively unstable compounds. Under hydrogen pressure, they are readily saturated to the paraffinic state. Indeed, three processes could be combined for the conversion of n-alkenes to isoalkenes, for example: first, hydrogenation of olefins into paraffins; second, skeletal isomerization of the paraffins; and third and finally, dehydrogenation of the skeletal paraffins into the desired iso-olefin. In this process combination, the first and third processes are accompanied by large heat effects and therefore may require several stages each; for light hydrocarbons, the conditions for the third process of the combination are usually quite severe.
Furthermore, in the presence of acidity, olefins can polymerize, crack and/or transfer hydrogen. Extensive polymerization would result in poor yields, and short operating cycles. Similarly, cracking would reduce yield. Hydrogen transfer would result in saturated and highly unsaturated compounds, the latter being the common precursors for gum and coke. Any theoretical one step process for producing skeletal isomers of, for example, n-butenes, would have to be concerned with the unwanted production of butanes and the reverse problem of production of butadienes. On top of all of these problems, it is well known that skeletal isomerization becomes more difficult as hydrocarbons get lighter.
Skeletal isomerization of olefins is known to be accomplished by contacting unbranched or lightly branched olefins with acidic catalysts at elevated temperatures. The process is generally applicable to the isomerization of olefins having from 4 to about 20 carbon atoms and is especially applicable to olefins having from 4 to about 10 carbon atoms per molecule. The process may be used to form isobutene from normal butenes, methyl pentenes and dimethyl butenes from normal hexenes, and so forth.
Known skeletal isomerization catalysts include aluminas and halogenated aluminas, particularly F- or Cl-promoted aluminas. Supports employed in such catalysts are either alumina or predominantly alumina due mainly to the high acidity of alumina. See Choudhary, V.R., "Fluorine Promoted Catalysts: Activity and Surface Properties", Ind. Eng. Chem., Prod. Res. Dev., 16(1), pp. 12-22 (1977) and U.S. Pat. No. 4,400,574. See also U.S. Pat Nos. 3,381,052, 4,405,500 and 3,444,096. Numerous catalysts employ a metal or metal oxide in conjunction with a halide-treated metal oxide. For example, U.S. Pat. No. 4,410,753 discloses isomerization catalysts comprising Bi.sub.2 O.sub.3 on fluorided alumina and U.S. Pat. No. 4,433,191 discloses skeletal isomerization catalysts comprising a Group VIII metal on halided alumina. U.S. Pat. No. 4,548,913 discloses catalysts for the skeletal isomerization of n-alkenes to iso-alkenes, containing admixtures of zeolites and clays which are preferably fluorine-treated.
Many of the catalysts including halide-treated components require periodic addition of halide materials to maintain catalyst activity; for example, see U.S. Pat. Nos. 3,558,734 and 3,730,958. Disadvantages of using such catalysts include the corrosion of process equipment due to the presence of halide ions and the necessity of adding halide materials as part of the feed as the reaction proceeds. Isobutene yields of 17 to 33 weight percent are typically reported when using halided catalysts, based upon a review of various patents cited in this disclosure.
Various techniques have been employed to improve the effectiveness of materials such as alumina and silica as skeletal isomerization catalysts. For example, U.S. Pat. No. 3,558,733 discloses methods for activating alumina catalysts with steam, U.S. Pat. No. 4, 405,500 discloses catalysts prepared by controlled deposition of silica on alumina and U.S. Pat. No. 4,587,375 discloses a steam-activated silicalite catalyst. In addition, various metal oxides have been used to improve the effectiveness of catalysts based upon alumina, silica or the like.
U.S. Pat. No. 4,654,463 discloses (in Claim 2) catalysts for the skeletal isomerization of olefins comprising bromided aluminas. Optionally, such alumina-based catalysts can include minor portions of various metal oxides, including, e.g. molybdenum and tungsten oxides. The active alumina and other metal oxides can be combined as physical mixtures or chemically bonded as in alumina-molybdena. Similarly, U.S. Pat. No. 4,778,943 discloses catalysts for the skeletal isomerization of olefins containing halogen-treated alkaline earth compounds, (all highly basic materials) optionally in conjunction with other metal oxides including alumina, molybdenum oxide and tungsten oxide.
The addition of sulfate ion to zirconia, alumina and other metal oxides has been reported to produce an enhancement in catalytic activities for acid-catalyzed reactions. It has been reported that sulfated alumina can isomerize 1-butene to 2-butenes but that no skeletal isomerization could be detected (see W. Przystajko et al in Applied Catalysis, Vol. 15, pp. 265-75 (1985). Recently been found to be active in the skeletal isomerization of n-butenes, and the addition of 8 weight percent sulfate to alumina provided a catalyst which produced "significant amounts of the skeletal isomer." However, the activity for isobutene production reportedly decreased by roughly 60 percent during the first ninety minutes of these runs. See J.C. Luy et al in React. Kinet. Catal. Lett., Vol. 36, pp. 275-79 (1988)
An object of this invention is an improved process for the skeletal isomerization of olefins, especially for the isomerization of n-butenes to form isobutene. A more specific object is an easily prepared, stable, active and selective isomerization catalyst and process for skeletal isomerization of olefins. Other objects and advantages of the invention will be apparent from the following description, including the drawing and the appended claims.